Institute of Electrical and Electronics Engineers (IEEE) 802.15.4e is an enhanced media access control (MAC) layer protocol of the IEEE 802.15.4 standard designed for low power and low data rate networks. The IEEE 802.15.4e architecture is defined in terms of a number of blocks in order to simplify the standard. These blocks are called layers. Each layer is responsible for one part of the standard and offers services to the higher layers. The interfaces between the layers serve to define the logical links that are described in the standard. A low-rate (LR)-wireless personal area network (WPAN) device comprises at least one physical layer (PHY) which contains the radio frequency (RF) transceiver (or radio) along with its low-level control mechanism, and a MAC sublayer that provides access to the physical channel for all types of transfers. The radio device can either transmit or receive at any given time, but cannot simultaneously perform both transmitting and receiving.
IEEE 802.15.4e is suitable for sensor devices with resource constraints; e.g., low power consumption, low computation capabilities, and low memory. As sensors and actuators that are interconnected by a PAN in home and office environments become more common, limiting the power dissipation for each device is important. Some radio devices may operate on a battery, in which case frequent battery changes or recharges are undesirable. Some other radio devices may operate on a limited amount of power that is generated by the device itself such as using conversion from solar or other light sources, energy scavenging from motion or thermal effects, or collection of energy from ambient electromagnetic fields.
Channel hopping wireless transmission system protocols typically have a retransmission mechanism to retransmit lost frames. When channel hopping is used, subsequent transmissions can use a different channel (frequency band) in the channel hopping sequence. This helps avoid channel interference that may have existed in the previous channel used causing frame loss so that channel hopping can improve network capacity. Channel hopping achieves increased network throughput by promoting simultaneous data transfer over multiple channels between different pairs of radio devices, or to achieve reliability in tough channel conditions by exploiting channel diversity.
Channel hopping can be achieved through many different known methods, with the most common methods used being either a synchronous method called Time Slotted Channel Hopping (TSCH) or an asynchronous method called un-slotted channel hopping (USCH) as defined in the IEEE 802.15.4e standard. Many standards also exist that use a channel hopping MAC to define MAC protocols for different applications. Standards also exist that use a channel hopping MAC to define MAC protocols for different applications. For example the Wi-SUN™ Alliance has published a Field Area Network (FAN) specification that specifies how to use asynchronous channel hopping for smart grid applications (Technical Profile Specification Field Area Network, Wi-SUN Alliance 2014, hereafter the “Wi-SUN FAN”).
In USCH (which does not require any synchronization for channel hopping) MACs, such as the one defined in the Wi-SUN FAN, frequency (or channel) hopping is achieved by a device by changing its receive (Rx) channel over different periods of time. The channel hopping sequence is based on a direct hash channel function (DH1CF) as defined in the Wi-SUN FAN specification which generates a pseudo-random sequence of channels based on the extended address of the node, and thus the channel sequence is unique to each node. Each radio device maintains its broadcast schedule as well as a unicast schedule. The radio device will transmit its broadcast data during its broadcast schedule and its neighboring radio devices who are already tracking this radio device are expected to be listening in its channel during the broadcast slot. During the broadcast interval, the radio devices follow their own receiver directed unicast channel hopping schedules. Being asynchronous channel hopping, the unicast channel hopping slots need not be synchronized to each other.
The unicast schedules are receiver directed in the sense that a radio device transmits the frame in the receiver radio device's current channel using carrier sense multiple access with collision avoidance (CSMA/CA). If the frame's data transmission goes beyond the slot period, the transmitting radio device then continues the data transmission into the adjacent slots of the receiver radio device as well. For example, responsive to a data request received from a radio device B, a radio device A may transmit a frame in radio device B's Rx channel and the data transfer from radio device A to radio device B may extend into the adjacent time slot.
There is network overhead needed to maintain proper synchronization in such USCH networks. The exchange of timing information requires the use of specific information elements (IEs) as defined in Wi-SUN FAN specification. There are two specific defined IEs in the Wi-SUN FAN specification. A Unicast Timing IE (UTT-IE) carries the timing information related to a hopping device node's Rx channel sequence needed for a fixed or semi-channel hopping device node to properly track the Rx hopping sequence, and a Unicast Schedule Information Element (US-IE) carries information regarding the channel number currently used by a fixed or semi-channel hopping device node (for generally sleepy device nodes) to the hopping device node.
The following example describes conventional USCH network operation per the Wi-SUN FAN specification. Assume a communication exchange between a coordinator hopping first radio device (first radio device) and a sleepy fixed or semi-channel hopping device node (sleepy radio device). A semi-channel hopping mode device does not channel hop on multiple channels continuously (even when not transmitting or receiving), where it instead hops to different channels only when transmitting or expected to be receiving a frame in response to its transmission of a poll (data request) frame.
The first radio device being a coordinator node can receive frames at any time and so will continue to keep its Rx on according to its hopping sequence. The hopping sequence is only for the hopping device node's Rx. When the first radio device being a hopping device decides to transmit, it will deviate from its Rx channel hopping sequence and transmit a frame on the target destination node's (sleepy radio device's) Rx channel and then return back to its Rx hopping sequence. The sleepy radio device is generally an end device in the network (e.g., a sensor network) that will receive a frame only as a response to its poll frame which it transmits after wake-up, so it does not have to keep its Rx always on.
The sleepy radio device operating on a fixed channel always expects data from the hopping device by an indirect transmission (as defined in IEEE 802.15.4 specification). A fixed radio device operating on a fixed channel will change its Rx channel of operation before every poll interval and may not follow any specific channel sequence. The first radio device being a hopping device changes its Tx channel based on channel information generally provided in a poll frame including a US-IE from the sleepy radio device. The sleepy radio device is able to track the timing of the Rx channel of the hopping device node based on an acknowledgement (ACK) frame with a UTT-IE from the hopping device node.
Thus to convey a sleepy radio device's current Rx channel to the first radio device being a hopping device a US-IE carried in a poll frame is used per the Wi-SUN FAN specification. To convey the first radio device's timing information related to its Rx channel hopping sequence to the sleepy radio device a UTT-IE is carried in all ACK frames and data frames per Wi-SUN FAN specification.